Liquid phase oxidation reactions of inorganic and particularly organic compounds using air or other source of molecular oxygen are important industrial chemical reactions. In these oxidation reactions the organic or inorganic compound undergoing oxidation is dissolved in a suitable reaction solvent, with or without an oxidation catalyst, and air or other oxygen-containing gas is typically sparged into the reaction mixture. However, for those organic compounds that are liquid at the oxidation reaction conditions a solvent is not always necessary. Utilization of air as a reactant to effect a chemical transformation has a readily apparent large economic incentive because the air need only be compressed and then injected or sparged into the reaction mixture. Consequently, when air can be used to selectively convert a given inorganic or organic compound to another useful compound, it is usually economically advantageous to do so. However, the transfer of oxygen from the vapor phase to the liquid phase is usually a critical part of any commercial liquid phase oxidation reaction. Mixing is the principal method for promoting the mass transfer of oxygen to the liquid phase in most liquid phase oxidation reactions where air or other forms of gaseous oxygen is the source of oxygen for the oxidation reaction. Many, if not all, of the problems associated with mixing a liquid phase oxidation reaction are present during the liquid phase oxidation of an alkylaromatic hydrocarbon to the corresponding aromatic carboxylic acid.
The liquid phase, heavy-metal catalyzed oxidation of an alkylaromatic hydrocarbon compound to the corresponding aromatic carboxylic acid is a reaction of particular commercial importance. Commodity aromatic carboxylic acids such as terephthalic acid, isophthalic acid and trimellitic anhydride are produced by one or more of such liquid phase oxidation processes in quantities of millions and, in some cases, billions of pounds per year. One such liquid phase oxidation process, the so-called Mid-Century Oxidation process, is a particularly suitable process for preparing aromatic carboxylic acids. In this process, as disclosed in Saffer and Barker, U.S. Pat. No. 2,833,816, a catalyst comprising cobalt, manganese and bromine components, a low-molecular weight aliphatic carboxylic acid reaction solvent and an oxygen-containing gas as a source of oxygen are used in combination to oxidize an alkyl group on an alkylaromatic hydrocarbon molecule to a carboxylic acid group. The oxygen-containing gas, typically air, is introduced into the reaction mixture and reacts with the alkylaromatic hydrocarbon in conjunction with the oxidation catalyst. Depending on the particular aromatic carboxylic acid produced by this process and on the conditions used for the oxidation reaction, the aromatic carboxylic acid may or may not be soluble in the reaction mixture. If insoluble or only partly soluble, the typically solid aromatic carboxylic acid will be present in the oxidation reaction mixture. Consequently, these liquid phase oxidation reaction mixtures always contain a gas phase and, in addition, may contain a solid phase as well.
In order to promote the contact of the oxygen-containing gas with the liquid phase components and, if present, to maintain the solid aromatic carboxylic acid suspended in the reaction mixture, these oxidation reaction mixtures are typically well mixed using a suitable agitator. Usually, these agitators comprise a rotating impeller located within the reaction mixture. Efficient mixing disperses the oxygen-containing gas thereby providing improved gas-liquid contact and improved mass transfer of the gas into the liquid phase where the oxidation reaction is occurring. For example, when mixing is inadequate, portions of the reaction mixture, so-called "pockets", have an insufficient concentration of oxygen. It is speculated that within these oxygen-deficient pockets the high molecular weight impurities or other reaction side products are formed that detract from the ultimate quality of the product aromatic carboxylic acid. Additionally, an increase in the concentration of oxygen in the liquid phase resulting from improved mass transfer and gas-liquid contact generally improves the rate of the oxidation reaction resulting in shorter reaction time and greater production rates for the aromatic carboxylic acid.
In known processes for the oxidation of alkylaromatics a specific, fixed mixing speed, i.e. the rotational speed of the agitator used for mixing the oxidation reaction, is selected. Speed selection is based on the estimated maximum density of the oxidation reaction mixture utilizing the accepted horsepower mixing equation: ##EQU1## Hp=mixing horsepower power no.=constant for a given system
density=density of reaction mixture or other composition being mixed PA1 rpm=agitator speed, revolutions per minute PA1 diameter=diameter of agitator impeller
Assuming the power number and impeller diameter remain constant it is apparent that mixing horsepower is proportional to the product of the agitator speed to the third power and the density value. Consequently, if the density of the reaction mixture varies while at a constant agitator speed, the horsepower delivered to the reaction mixture varies accordingly.
During the oxidation of an alkylaromatic hydrocarbon to a carboxylic acid the density of the reaction mixture does change. Change in density arises due to factors such as changing reaction temperature, pressure, planned changes in the rate of addition of the oxygen-containing gas, and due to the production of the oxygenated aromatic carboxylic acid product. Furthermore, there may be a fluctuation in reaction mixture density due to changes in the rate of addition of the oxygen-containing gas to the reaction mixture as may be caused, for example, by inadvertent fluctuations in the output of the compressor used for supplying the oxygen-containing gas to the oxidation reactor. Under conditions where the oxygen-containing gas is being introduced to the reaction mixture, commonly referred to as the gassed state, the density of an oxidation reaction mixture may be only one half of the density of the reaction mixture in the ungassed state. Even when fully gassed, an oxidation reaction mixture may vary in density by as much as 7% from the beginning of the oxidation reaction to the end of the reaction due to the aforementioned factors such as reaction mixture composition, reaction temperature, and scheduled changes in the rate of addition of the oxygen-containing gas.
Due to the use of a fixed agitator speed in prior processes and due to the changing density of the alkylaromatic hydrocarbon oxidation reaction mixture, the agitator speed is set in prior processes using the maximum estimated density of the reaction mixture. Otherwise, assuming full utilization of the capacity, i.e. fully loaded condition, of the mixing motor used for rotating the agitator, the motor would overload when the reaction mixture density increases. Particularly drastic results would occur if the gas flow were to greatly decrease or stop since this would produce an abrupt change in density and, consequently, a severe overload on the motor. Furthermore, if the motor overloads and mixing stops product quality may be greatly reduced requiring the disposal of the contents of the reactor. Potentially explosive conditions could also develop.
Therefore, when employing the standard, fixed agitator speed method of mixing a liquid phase oxidation reaction, full mixing capacity is not utilized throughout the course of the oxidation reaction. During the phase of the oxidation reaction where the density is below maximum, the power delivered to the oxidation reaction is below the maximum available or, in other words, the mixing speed is below that which is possible at that particular reaction mixture density.
A new method for increasing the efficiency of a liquid phase oxidation reaction and, in particular, the oxidation of an alkylaromatic hydrocarbon to an aromatic carboxylic acid by increasing the efficiency of the mixing of the oxidation reaction mixture would be highly advantageous. The present invention provides such a method.